The coloration extinction boundary is
poorly defined near threshold coloration;
only a few very light bands in biotite could
be reliably measured. Reproducible
measurements were obtained in the plateau
region (14), where variations in band size
are minimal. Darker halos in biotite
generally have slightly larger radii than
lighter halos (3, 4). Also, reversal effects in
some biotites immediately exterior to the
terminus of a halo ring cause apparent
diminution of the radius. Therefore, while
there are differences between the sizes of
medium coloration hands (Table 1, column
2) and the radii of U halos in biotite (Table 1,
columns 8, 9, and 10) that could be
interpreted in terms of an actual change in
Eα and λ (16), such differences more likely
arise from a combination of dose and
reversal effects (15, 17), producing slightly
diminished radii. Diminution of U halo
radii may also result from attenuation of α-particles within the small but relatively
dense zircon radiocenters. Even though
slight differences between band sizes and
U halo radii do exist in biotite, the
idealized U halo ring structure (Fig. 1a)
compares very well with an actual U halo
in biotite (Fig. 1f).

Biotite and fluorite are good halo
detectors, but fluorite is superior because
the halo rings exhibit more detail, often
have smaller radiocenter diameters (< 1
μm), and have almost negligible size
variations due to dose effects in the
embryonic to normal stages of
development. Figure 1g shows an
embryonic U halo in fluorite with only the
first two rings fully developed; the other
rings are barely visible because, due to the
inverse square effect, threshold coloration
has not been reached. Figure 1h shows a U
halo in fluorite in the normal stage of
development, when nearly all the rings
are visible. This halo closely approximates
the idealized U halo in Fig. 1a. Under high
magnification even separation of the 210Po
and 222Rn rings may be seen. Figure 1i
shows another U halo in fluorite, with a
ring structure that is clearly visible but not
adequate for accurate radius
measurements.

In Table 1, columns 4, 11, and 12, the
fluorite band sizes agree very well with the
U halo radii measured in this mineral by
myself and Schilling (9). This suggests that
the differences between U halo radii and
band sizes in biotite are not due to a
change in Eα However, experimental
uncertainties in measuring U halo radii
preclude establishing the constancy of λ to within 35
percent, and under certain assumptions U
halos provide no information at all in this
respect (16).

While halos with point-like nuclei which
show well-defined, normally developed
rings (as in Fig. 1h) can be used to
determine the Eα's of the radionuclides in
the inclusion, there are pitfalls in
ascertaining what constitutes a normally
developed ring. In contrast to the easily
recognizable U halos in fluorite in Fig. 1, g
to i, the overexposed fluorite U halo in
Fig. 1j shows a diminutive ghost inner ring,
which could be mistaken for an actual 238U
ring. Figure 1k shows two other partially
reversed U halos, one of which shows the
diminutive inner ring, while in the other all
the inner rings are obliterated. The U halo
in Fig. 1l is even more overexposed, and
encroaching reversal effects have given
rise
to another ghost ring just inside the
periphery. Figure 1m shows a still more
overexposed U halo; in which second-stage reversal effects have produced
spurious ghost rings that are unrelated to
the terminal α-particle ranges.

Since this association of the halos in Fig.
1, l and m, with U α-decay cannot be easily
proved by ring structure analysis alone, I
have utilized electron-induced x-ray
fluorescence to confirm this identification.
Figure 3a shows the prominent Ca x-ray
lines of the fluorite matrix (the F lines are
below detection threshold) along with
some background Ag and Rh lines which
are not from the sample, but are produced
when back-scattered electrons strike a Ag-Rh alloy pole piece in the sample chamber.
Figure 3b, the x-ray spectrum of a halo
radiocenter typical of the halos in Fig. 1, l
and m, clearly shows the x-ray lines due to
U (as well as a small amount of Si) in
addition to the matrix and background
peaks. A more detailed analysis (18)
reveals that the Uζ line masks a small
amount of Pb probably generated by in situ
U decay.

The variety of U halos shown in Fig. 1, g
to m, establishes two points: (i) only a thorough search will reveal the
numerous variations in appearance of U
halos, and (ii) unless such a search is
made, the existence of halos originating
with α-emitters other than 238U or 232Th
could easily be overlooked.

So far, three criteria have been used to
establish the identity of U halos: (i) close resemblance of actual halos in
biotile (Fig. 1f) and fluorite (Fig. 1h) to the
idealized ring structure [p. 242]
(Fig. 1a), (ii) identification
of lines in x-ray fluorescence spectra,
and (iii) agreement between U halo radii
and equivalent band sizes (very good in
fluorite and fair in biotite and cordierite).
Using the third criterion (either band sizes
or U halo radii) I can determine Eαfor a
normally developed fluorite halo ring to
within ± 0.1 Mev. For biotite halos, U halo
radii may form a suitable standard for
determining Eα for rings that show reversal
or other effects characteristic of U halos in
the same sample. If good U halos are not
available, and if the halos with variant sizes
show well-developed rings without reversal
effects, then the band sizes form a suitable
standard for Eα determination when
coloration intensities of variant halos and
band sizes are matched.